Application of Biochar for Cadmium Stabilization in Contaminated Paddy Soil

Cadmium contamination in rice fields near zinc mines in Mae Sot District, Tak Province has been a persistent problem for decades. The contamination covers a vast area, making several soil remediation methods, such as soil washing or excavation impractical. Phytoremediation would also take several years and interfere with farming. However, there are recent reports on the potential of biochar in cadmium stabilization which resulted in reduced cadmium uptake by rice. Therefore, in this study, several biomaterials were investigated to produce the most suitable biochar for cadmium stabilization in paddy soil in Mae Sot. Rice husk, rice straw and bagasse were selected because of their availability in Mae Sot. Cadmium adsorption efficacy of biochar made from these biomaterials were analyzed using the adsorption isotherm. The results showed that cadmium adsorption by biochar from bagasse, rice husk, and rice straw were in accordance with Freundlich adsorption isotherm equation. By comparing the distribution coefficient, the adsorption efficacy of the biochar was ranked as rice straw> bagasse> rice husk. Therefore, rice straw biochar, which had the highest cadmium adsorption efficacy, was selected for stabilization experiments. After 30 days of incubating contaminated soil with rice straw biochar, the amount of extractable Cd using CaCl2 and EDTA were reduced significantly. The optimal application rate of rice straw biochar was 5% with the incubation period of 20 days. This warrants the next phase of this study which will be conducted in the field experiments in Mae Sot